NUCLEOTIDE-BINDING OF THE C-TERMINAL DOMAINS OF THE MAJOR HISTOCOMPATIBILITY COMPLEX-ENCODED TRANSPORTER EXPRESSED IN DROSOPHILA-MELANOGASTER CELLS

The C-terminal domains of the mouse transporter associated with antige n processing (TAP) were expressed as soluble proteins in Drosophila me lanogaster cells and labeled by [alpha-P-32]8-azido-ATP after UV-irrad iation. The relative potencies of the nucleotides in preventing azido- ATP labeling were in the order of ATP > GTP > CTP > ITP > UTP for both the TAP1 and TAP2 C-terminal domains, suggesting ATP to be the natura l substrate of the transporter. Our data provide the first evidence th at the individual C-terminal domain of either TAP1 or TAP2 can be expr essed as a functional ATP-binding protein.